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Using the state of reefs for Anthropocene stratigraphy: An ecostratigraphic approach

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(Excerpts from text) … Reefs as ecosystems have shown pronounced evolutionary changes as well as marked adaptation to distinct but changing ecological reef settings, and they hence were prone to reflect large-scale synchronous global impacts as well as regional ecological changes. Therefore, extensive episodes of global reef decline during major Earth-history global extinction events, followed by millions of years of reef recovery times, provide excellent global correlation markers. … Although most present tropical reefs are dependent on photosymbiosis, letting them flourish within a narrow range of warm, superoligotrophic (with low nutrients and high dissolved oxygen) waters, there are still some modern reefs that, from their organic composition and environmental adaptations, resemble geologically much older reefs and hence are termed ‘atavistic’ Such reefs are composed of more robust, more resilient, more adaptable or even ‘Lazarus’-type relic taxa, which allow them to thrive in high-sediment and - nutrient settings, deeper waters and generally unstable environmental regimes. Atavistic reefs and Lazarus-reef taxa can be expected to become more common as the Anthropocene progresses … Ecostratigraphic Scenario 1 … Despite many efforts to support coral reefs, there is general scientific agreement that under ‘business as usual’ conditions there will be a global demise or severe reduction of coral reefs somewhere between the middle and end of the 21st century. … Earth history is no comfort in this respect, because recovery following global shallow- water reef extinctions during the Phanerozoic took place over many millions of years Ecostratigraphic Scenario 2: … Some reef researchers see the evolutionary and epigenetic adaptivity of corals and other reef organisms as much larger than previously thought, believing that especially in a combined effort of (1) keeping atmospheric CO2 below 450 ppm, (2) extensive management of reefs (in relation to overfishing, eutrophication, sediment runoff and other pollution as well as tourist or shipping damage) via marine parks, and (3) ‘assisted’ approaches to enlarge resilience of coral reefs, there might be progress towards enhancing coral-reef development during the future Anthropocene … under this low- emission scenario, coral reefs would experience an increase of SSTs from 0.30°C to 0.68°C within the period from 2010 to the end of the 21st century. The entire range of temperature increase, including the past century, would still be near or above 1°C, which is considered to be critical to coral reefs, but at least some of them could continue to exist. … Living with the vale of tears: The future of Anthropocene coral reefs relies on reducing emissions of CO2 from fossil fuels and on new forms of governance, management and educational concepts to limit the human impact on coral reefs. However, it may also rely on protecting and supporting exceptional, in part atavistic reef types that so far thrive only in places different from the classic modern oligotrophic reef settings and can grow under elevated sediment and nutrient influx … reefs will only have a chance if (a) strong management (including new protection and social-management schemes, as also outlined by Hughes et al. 2017b) is implemented and (b) emissions from fossil fuels stop around 2050. But even in this positive case, reefs would have to cross a critical ‘vale of tears’ phase (Figure 3.4.3b). Such specialised reefs would only have a chance to survive if allowed to adapt to the new conditions in a natural way or helped via human- assisted adaptation. Such reefs would be of lower diversity, having fewer ecological niches, and would probably be short-lived and patchy. They would most likely not play an important role in coastal protection and possibly only a reduced role in supporting fish stocks. However, after such a phase of volatility and adaptation, reefs might hopefully recover in the late 22nd or 23rd century, possibly forming large, structured reefs again, though different in taxon structure relative to Holocene ones.
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3.4 Using the State of Reefs
for Anthropocene Stratigraphy:
An Ecostratigraphic Approach
Reinhold Leinfelder
3.4.1 General Considerations
Throughout Earth history, reef organisms have only
minor potential for biostratigraphic use, with
exceptions such as some Paleozoic rugose corals and
several Cretaceous reef-building rudist bivalves.
However, reefs as ecosystems have shown
pronounced evolutionary changes as well as marked
adaptation to distinct but changing ecological reef
settings, and they hence were prone to reect large-
scale synchronous global impacts as well as regional
ecological changes.
Therefore, extensive episodes of global reef decline
during major Earth-history global extinction events,
followed by millions of years of reef recovery times,
provide excellent global correlation markers.
Although the causes for these global extinctions are
not all fully understood, elevated sea surface
temperatures (SSTs) combined with a strong lowering
of marine seawater alkalinity and sometimes turnover
of oceans into an oxygen-decient state appear to
be associated with most, if not all, extinction events.
In most cases extensive volcanism (at times perhaps
triggering additional methane clathrate degassing and
blowouts from ocean sediments) was related to the
extinction events, with additionally an asteroid
impact being substantiated for the Cretaceous/
Paleogene boundary. Reef recovery rates were very
slow, of a minimum duration of a few million years
and a maximum of 140 million years in coral-rich,
open-water tropical reefs, extending from the
midLate Devonian extinction to the Late Triassic
onset of shallow-water scleractinian reefs
(Figure 3.4.1). Other reef types, such as richthofeniid
reefs, muddy reefs with a low-diversity fauna of
corals or deeper-water mounds rich in phylloid algae,
siliceous sponges, calcisponges and bryozoans
continued to grow during this extensive gap. This
might also be important for reassessing reef growth
during the recent episode of Anthropocene reef crisis
(Stanley 2001a, b).
1
Less often used for stratigraphic purposes are
shorter-term changes in the growth episodes,
architecture and community structure of reefs, as
controlled by sea-level change. Reef expansion
episodes are most typically associated with relative
sea-level rises. During times of high sea level, reefs
may be covered by other sediments, while when sea
levels are low, reefs are often exposed subaerially and
cease to grow. Such dynamic patterns can be highly
signicant for deciphering sea-level changes lasting
~0.5 to 3 Ma or less, governing available space for
reef growth, wave energy level and the inux of
siliciclastic sediments and nutrients from terrestrial
areas. The last of these may result in regional shallow-
water oxygen depletion and cessation of reef growth.
This relationship has made reefs an important
ecostratigraphic tool for constraining sea-level
history in ancient tropical marine environments,
especially during the Mesozoic, where the new
scleractinian coral reefs thrived in a great variety of
settings next to other reef types such as siliceous
sponge reefs and microbial reefs, each having its
distinct environmental framing. The dependence of
reef organisms and reef types on particular
environmental settings does not preclude using them
for stratigraphic correlation, so long as there is a good
appreciation of controls on reef parameters and
how to model them in an ecostratigraphic context
(Hofmann 1981; Sokolov 1986; Oloriz et al. 1995).
Consequently, ancient reefs have been used to
constrain global sea-level histories with generally
good results. Their geometric growth patterns,
1
For overviews of reef evolution and extinctions through Earth
history, see Wood, (1999), Leinfelder and Nose (1999), Stanley
(2001a, b) and Veron (1995, 2008); for global extinction
events, see also Buggisch (1991), Copper (2001), Flügel and
Senowbari-Daryan (2001), Hautmann (2012) and Clarkson
et al. (2015).
128 3.4 Using the State of Reefs for Anthropocene Stratigraphy: An Ecostratigraphic Approach
Personal copy (proof, with final corrections included)
Published in: Jan Zalasiewicz, Colin Waters, Mark Williams, Colin Summerhayes, (eds), (2019): The Anthropocene as a Geological
Time Unit. A Guide to the Scientific Evidence and Current Debate, Cambridge University Press, ISBN 9781108475235
have had only
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Figure 3.4.1 The past, present and possible future of reefs.
(a) Coral-rich reefs through time. Growth episodes (highs), major Earth-history extinction events (red labelled lines),
reef recovery times (red rectangles); long horizontal line is the 140-million-year-long gap of tropical, coral-rich reefs.
Numbers are millions of years. Precambrian not to scale. Modied after Veron (2008).
(b) The concept of increasing overall reef complexity, based on subsequent addition of reefal building blocks, which persisted
throughout Earth history. Note that modern tropical coral reefs still contain microbial crusts within reef caves and include
demosponges as well as calcifying sclerosponges (based on Leinfelder & Nose 1999).
(c) After a number of changes from the Proterozoic (stromatolite reefs) to the Mid-Paleozoic (possibly already
including photosymbiontic reef organisms, chiey stromatoporoids and some tabulate corals) and the long gap of tropical
coral-rich reefs from the latest Devonian to the Late Triassic, the width of reef windows, as dened as the maximum
available space of reef settings, became probably largest during the Jurassic, with microbialite reefs and a great variety of
different siliceous sponge reefs and tropical coral reefs co-occurring. After optimising the photosymbiontic system and with
the onset of coralline algae conquering high-energy settings, modern tropical coral reefs are in a much narrower,
more superoligotrophic window than, e.g., Jurassic coral reefs. For the Anthropocene, a best-case scenario is shown, with
reef compositions changing and relic reef types that are adapted to higher nutrients, more runoff, elevated temperatures
and reduced alkalinity reoccurring in a volatile fashion. The widening of the Anthropocene reef window (which is not
reecting size and frequency of reefs) corresponds to the ecostratigraphic reef episodes (15b) as outlined in Figure 3.4.3.
Modied from Leinfelder et al. (2012).
The Biostratigraphic Signature of the Anthropocene 129
(vertical labelled lines),
(black rectangles); bold
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composition, indicators for shifts in terrigenous
runoff or even oxygen concentration, can be used to
establish different patterns of reef growth. This in turn
allows the detection of sea-level uctuations and
enables their use for chronostratigraphic purposes
(e.g., Sarg 1988; Schlager 1992; Leinfelder 1997,
2001; Leinfelder & Wilson 1998; Leinfelder & Schmid
2000; Leinfelder et al. 2002).
It is only in Cenozoic reefs that direct proxies for
sea surface temperature (SST) can be derived from
oxygen and Ca/Sr isotopes (Figure 3.4.2) and seawater
alkalinity from boron isotopes (for pH) (see Section
5.3). Such proxies are recorded in the annual skeletal
growth bands, which can be either directly dated by
14
C measurements, dated by backward counting of
growth rings, or correlated via characteristic waxing-
waning sets of growth rings, as done with tree rings.
SSTs appear to be recorded with a mostly negligible
vital effect on isotopic composition within the
aragonitic skeleton. Nevertheless, there are
restrictions: oxygen isotopes are particularly
dependent on salinity and might also need other
corrections, such as the amount of isotopically light
water being bound in polar ice caps, while the possible
effects of diagenesis also need to be taken into
account. For coral skeletons, stable isotopes are the
key instruments for identifying global or regional
shifts in seawater temperature for the Cenozoic,
chiey in the Neogene, and may also be used as
stratigraphic tools (e.g., Fairbanks & Matthews 1978;
Ahmad et al. 2011; Chappell & Shackleton 1986;
Zinke et al. 2014; Tierney et al. 2015; DeLong et al.
2016).
Although most present tropical reefs are dependent
on photosymbiosis, letting them ourish within a
narrow range of warm, superoligotrophic (with low
nutrients and high dissolved oxygen) waters, there are
still some modern reefs that, from their organic
composition and environmental adaptations,
resemble geologically much older reefs and hence are
termed atavisticby Leinfelder et al. (2012). Such
Figure 3.4.2 Comparison of SST records between the
years 1600 and 2000 CE as reconstructed from coral
skeletons (mostly using
16
O/
18
O isotope and Ca/Sr ratios)
from different tropical reefal settings, allowing
correlation of SSTs across oceans. Shown are averages
within the ENSO (three to seven) year band, (a)(d); (e)
shows a comparison with instrumental data (from
Tierney et al. 2015).
130 3.4 Using the State of Reefs for Anthropocene Stratigraphy: An Ecostratigraphic Approach
Sr/Ca
Sr/Ca
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reefs are composed of more robust, more resilient,
more adaptable or even Lazarus-type relic taxa,
which allow them to thrive in high-sediment and -
nutrient settings, deeper waters and generally
unstable environmental regimes. Atavistic reefs and
Lazarus-reef taxa can be expected to become more
common as the Anthropocene progresses (see
Section 3.4.3).
3.4.2 The Present Situation of
Tropical Reefs with Regard to
Stratigraphic Correlation
Despite todays narrow superoligotrophic reef
window, Quaternary tropical coral reefs are basically
resilient ecosystems that can tolerate or easily recover
from occasional temporary disturbances. Damage
caused by a tropical cyclone or coral bleaching due to
temperature peaks is reparable if sufcient
regeneration time is available before a new
disturbance occurs, so long as the overall ecological
conditions for reef growth are in order. As in the
grazing of savannas, an occasional storm may even
foster biodiversity by providing fresh substrata to
larvae of slower-growing corals. Other corals, such as
some acroporoids, even use fracturing for vegetative
reproduction and more extensive dissemination, with
coral branch fragments becoming cemented to the
ground by surviving polyps and restarting colonial
growth. Such natural resilience is rapidly diminishing
in most recent coral reefs. Overshing, water
pollution (owing to runoff of soil, fertilisers, other
chemicals, plastic particles, etc.), global warming and
the decreasing pH of higher-latitude tropical marine
waters have dramatically increased the vulnerability
of reefs (e.g., Wells & Hanna 1992; Hoegh-Guldberg
2007; Heiss & Leinfelder 2008; Burke et al. 2011;
Schoepf et al. 2015; Hughes et al. 2003, 2017a).
Hence, natural perturbations, such as El Niñotype
temperature peaks (discussed in Section 6.1), have
stronger and much more widespread ecological
effects. It is highly probable that the increase in the
frequency and intensity of both high-temperature and
storm events is caused by the anthropogenic rise of
atmospheric CO
2
(e.g., GFDL 2017; Reed et al. 2015).
In addition, rainfall associated with tropical cyclones
may result in pulses of strongly increased inux of
soil particles, nutrients and pesticides from agriculture
in the hinterland, as frequently happens on the Great
Barrier Reef (Brodie et al. 2013). This results in
eutrophication peaks in the oligotrophic tropical reef
ecosystem, leading to blooms of planktic algae and
severe overgrowth of corals by soft lamentous
benthic algae. Owing to the overall weakening of the
reef system by these factors, corals are more
vulnerable to natural diseases. Stressed coral reefs
have difculty in recovering, especially if individual
events such as hurricanes or bleaching events coupled
with temperature peaks occur too often. In a
stratigraphic context, ecological perturbation events
in coral reefs can affect vast areas and occur
penecontemporaneously in different regions or even
globally across the entire tropical zone, and hence
they should be stratigraphically correlatable.
After severe bleaching events in 1997/1998,
2002 and partly in 2010, 2016 was one of the
strongest bleaching events in the Great Barrier Reef
(GBR). About 90% of the surveyed 1,156 individual
reefs of the GBR complex were affected, and >60% of
all corals were bleached. There was no recovery time,
because another devastating bleaching related to a
renewed El Niño event took place in 2017, possibly
representing the most pronounced coral bleaching in
the history of the GBR. It is reported that about 70%
of the shallow-water corals around the tourist town
of Port Douglas died, with similar values around
Cairns and Townsville (Hughes et al. 2017a; GBRMPA
2017). In addition, the devastating effects of Cyclone
Debbie, which swept across Australia in March
2017, not only smashed many reef regions into rubble
but once again swept mud, fertilisers and pollutants
into the reef regions (Robertson 2017). Hence, as with
earlier bleaching events, especially the 1997/1998
event, the 2016 bleaching event can be correlated not
only across the major part of the GBR (e.g., Cantin &
Lough 2014) but also across the Indian Ocean, other
The Biostratigraphic Signature of the Anthropocene 131
event took place in 2017, possibly
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parts of the Pacic and the Caribbean. There are
many studies on how bleaching events are recorded
not only in the SST-recording isotopic oxygen
signal but also in growth patterns or isotopic
proxies of reduced calcication (e.g., Pereira et al.
2015; DeLong et al. 2016; DeCarlo & Cohen 2017), but
using this for interregional to global time-slice
correlations based on coral skeleton characteristics
and proxies is still in its infancy (e.g., Neukom et al.
2014; Tierney et al. 2015; Abram et al. 2016;
Figure 3.4.2 herein).
3.4.3 The Application of Reef Stratigraphy:
Ecostratigraphic Scenarios for the
Anthropocene
3.4.3.1 Ecostratigraphic Scenario 1:
Correlating Reefs in Deteriorating Ecological
Settings (Figure 3.4.3a)
Since stone corals are good recorders of SSTs through
possessing annual skeletal growth bands and
mirroring SSTs in equilibrium conditions via oxygen
or Sr/Ca-isotopes, they can monitor rising SSTs as
long as they are not killed by the heat (e.g., Ahmad
et al. 2011; Tierney et al. 2015; DeLong et al. 2016).
Despite recording only local temperatures, such
measurements can be correlated using growth-ring
counts or other characteristics, such as waxing/
waning patterns of sets of growth rings or even
14
C ages. SST-peak-related bleaching events, as
discussed above, can be correlated across the entire
globe via combining SST proxies and micro-erosion
events across reefs. Storms, if pronounced, especially
as recovery times now are slow, should also be
correlatable across wide areas of reefs via disruptive
surfaces associated with a dominance of reef rubble
within the reefs and lagoonal settings. Overshing
and eutrophication of reefs are resulting in a strong
reduction of reef coral diversity, overgrowth by algal
turfs (which can be fossilised as microbored surfaces),
or macroborings by organisms such as sponges and
bivalves. Interruptions of reef growth, caused by
coastal runoff, may also be discernible and probably
correlatable via cessation of reef growth in
association with biological and physical erosion, as
well as via coatings of terrigenous material.
If anthropogenic greenhouse gases continue to be
emitted at the current rates, even strong attempts at
local or regional reef management or reef protection
will not help prevent highly diverse and structured
coral reefs from disappearing, possibly as early as the
mid-21st century. In such scenarios, ocean
acidication spreading from higher latitudes towards
the equator will prevent coral reefs escapingpoleward
from rapidly rising tropical SSTs (e.g., Hoegh-Goldberg
et al. 1999, 2007, 2009; Hughes et al. 2017a, b).
Escapeto deeper, somewhat cooler waters would also
be largely hindered by these being too turbid because
of the increasing runoff of nutrients creating more
plankton. Nevertheless, it is important to recognise that
coral reefs have survived frequent warmings and
coolings, most of them not rapid, within the
Pleistocene. Furthermore, it is clear that the same
corals that populate the Great Barrier Reef also exist
on the much warmer reefs around New Guinea.
Hence we cannot predict with condence that
present-day reefs will die out completely as the oceans
continue to warm. Given sufcient time, reef
organisms may be able to adapt, although the centres
and patterns of reef growth may well shift in time as
the climate changes.
Relating this reactivity of reefs to an Anthropocene
under business as usualconditions (in terms of
climate change and environmental pollution) would
mean that ecological change of coral reefs relative to
the earlier Holocene should, at least to some extent,
be detectable from the 15th century onwards.
Holocene reefs likely began to transform since
Columbian times (Jackson 1997), especially due to the
onset of intensive shing, often resulting in local to
regional overshing, including that of sea turtles and
sea mammals. These early changes may characterise
the initial anthropogenic imprint on reefs, expressed
in the form of reduced diversity and coral coverage
and subsequently increasing in response to the effects
of growth in population, world trade and
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industrialisation during the mid-20th century
(see Section 7.5).
The suggested base of the Anthropocene around the
mid-20th century is expected to be correlatable across
coral reefs (see Section 7.8.4.2 for use of corals as a
potential medium for the placement of a GSSP) by the
following:
(1) Spikes of radioactive fallout from nuclear bomb
tests preserved in coral skeletons, beginning in the
early 1950s, peaking in the 1960s and continuing
until the 1980s (Waters et al. 2016)
(2) Possibly also enrichment of Pb from industrial
activity, since corals are able to accumulate heavy
metals (e.g., Berry et al. 2013)
(3) An increase of plastic and other anthropogenic
particles trapped in interstitial reef voids and
cavities from the 1950s to present
Younger ecostratigraphic correlation within reefs
might be possible by the following:
(1) The nearly complete disappearance of sea urchins
owing to a pervasive infection in the Caribbean
in the 1980s (Knowlton 2001)
(2) A strong decrease in coral-reef diversities since the
late 1980s/early 1990s, especially with the strong
reduction to near disappearance of Caribbean
acroporoid corals, paralleled by an increase of lter
feeders and boring organisms (e.g., Seemann et al.
2012; Lirmann et al. 2014)
(3) Correlation of the 1997/1998, 2002 and 2016
global bleaching events, using coral skeletons (see
Figure 3.4.2)
(4) Correlation of the onset of invasive species, such
as occurrences of teeth and other skeletal parts
of the lion sh in the Caribbean in the early 21st
century (Schoeld 2009), possibly preservable in
muddy reef lagoons
Despite many efforts to support coral reefs, there is
general scientic agreement that under business as
usualconditions there will be a global demise or severe
reduction of coral reefs somewhere between the middle
and end of the 21st century (Figure 3.4.3a), providing
another important biostratigraphic marker horizon. As
pointed out above, Earth history is no comfort in this
respect, because recovery following global shallow-
water reef extinctions during the Phanerozoic took
place over many millions of years (see Section 3.4.1).
3.4.3.2 Ecostratigraphic Scenario 2:
Using AssistedCoral-Reef Episodes
of the Anthropocene
Some reef researchers see the evolutionary and
epigeneticadaptivity of corals and other reef organisms
as much larger than previously thought, believing that
especially in a combined effort of (1) keeping
atmospheric CO
2
below 450 ppm, (2) extensive
management of reefs (in relation to overshing,
eutrophication, sediment runoff and other pollution as
well as tourist or shipping damage) via marine parks,
and (3) assistedapproaches to enlarge resilience of
coral reefs, there might be progress towards enhancing
coral-reef development during the futureAnthropocene
(see also Section 3.3). There are presently many studies
in the new eld of assisted adaptabilityaiding the
evolution of coral reefs. For example, recovery after
mechanical reef damage, such as tropical storms or the
collision of a ship on a reef, could be assisted by
replanting cultured coral (for an overview see Ferse
2008), although costs would be extremely high on a
larger scale, and long-term success has not yet been
demonstrated. Some coral species are potentially better
adapted to higher water temperatures (Schoepf et al.
2015) or minor acidication (Shamberger et al. 2014),
but these cannot be transplanted into other regions so
far, a restriction which is not fully understood yet. Some
working groups experiment on assisted evolutionin
order to breed resistant species (e.g., van Oppen et al.
2015), but despite some local success (e.g., Zayasu &
Shinzato 2016), so far there have been several setbacks
(Hughes et al. 2017b). Hence the exact temporal onset of
a recovery phase through allowing and enhancing
adaptation and dissemination of substitute corals is
difcult to predict, especially since many factors have to
be taken into account. Here, three aspects are
considered:
The Biostratigraphic Signature of the Anthropocene 133
2017a). Hence
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Figure 3.4.3 Use of Anthropocene coral reefs for ecostratigraphic purposes. Shown are two conceptual scenarios.
(a) Business as usual(BAU) scenario (relative to anthropogenic atmospheric CO
2
emissions); (b) an integrated CO
2
-mitigation/
reef-management/assisted-adaptation scenario. Based on two scenarios taken from Hoegh-Guldberg et al. (2009), as
developed for the Coral Reef Triangle, adapted and extended. In the BAU scenario (a), where atmospheric CO
2
will keep rising,
not even strong management (with or without assistedadaptation) will help coral reefs. The combined mitigation/
management/adaptation scenario (b) will also not see a rapid reversal of the negative trend, but after going through a vale of
tears, it might eventually see the return of fully developed coral reefs.
Ecostratigraphic episodes:
a+b:
(1) Post-Colombian coral-reef phase (pre-Anthropocene, ~1492 to 1950 CE)
(2) Early Anthropocene coral-reef phase (~1950 to 2000 CE), showing increasing, often punctuated decline of coral reefs.
Further subdividable by ecocrisis events such as the near disappearance of Diadema sea urchins in the Caribbean or severe
global bleaching events (dotted line).
(3) Deterioration phase (from 2000 CE onwards): accelerated decline of coral reef, with more frequent punctuation events (e.g.,
2016 bleaching event, dotted line) and rapid deterioration of coral coverage and coral reef occurrences.
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(1) Assisting partial adaptation towards elevated
SSTs and reduced alkalinity : Hughes et al. (2017b)
suggest that whereas scenarios for business as usual
on SSTs and shallow-water ocean acidication
threaten living coral reefs with no chance of survival,
the prognosis would be very different under a
mitigation scenario. SSTs would still continue to rise
slightly in the short term (2010 to 2039 CE), even if
global emissions began to fall, but this would be only
in the range of 0.32
!
C to 0.48
!
C. From 2039 to 2099
CE, SSTs would begin to stabilise, with temperatures
changing, depending on different reef provinces, from
+20
!
C to -0.05
!
C. Consequently, under this low-
emission scenario, coral reefs would experience an
increase of SSTs from 0.30
!
C to 0.68
!
C within the
period from 2010 to the end of the 21st century. The
entire range of temperature increase, including the
past century, would still be near or above 1
!
C, which
is considered to be critical to coral reefs, but at least
some of them could continue to exist. There is
evidence that the geographical range of tropical
species does not contract towards the equator, owing
to acidication, but rather some species expand
towards the subtropics, eeing increases in SST
despite having to handle slight decreases in aragonite
concentration (Pandol2015; Poloczanska et al.
2016). After the mass bleaching event of 1997/1998 in
the Maldives, the reefs changed their composition to
some extent. The more temperature-resistant and
robust scleractinian coral Pavona varians survived
better than Montipora and spread rapidly at the cost
of other species (Loch et al. 2002, 2007). Other corals,
such as Acropora hyacinthus from the Pacic
volcanic island Ofu, can withstand temperatures of up
to 38
!
C by activating special genes (Barshis et al.
2013). However, heat-tolerant corals cannot simply be
transferred from one site to another, as shown, e.g., by
Polato et al. (2010) for larvae of the Caribbean star
coral Montastrea faveolata. Yet the possible exchange
of photosymbionts to more heat-tolerant types (e.g.,
Rohwer & Youle 2010), such as the recently
discovered Symbiodinium C-type S. thermophilium,
which can withstand up to 36
!
C as a symbiont (Hume
et al. 2015), together with a much better
understanding of comparative genomics of reef corals
(e.g., Bhattacharya et al. 2016), gives some hope that
the original, frequently criticised hypothesis of
adaptive coral bleachingmight have at least partial
applicability (Buddemeier & Fautin 1993; Buddemeier
et al. 2004). Hughes et al. (2017b) suggest that provided
a full integrated management of other stressors, such
as overshing and pollution, is introduced by using
new integrated heuristic models, which include
socioeconomic drivers, and provided coral reefs are
allowed and assisted to change their composition, there
may be a chance that functional reefs, albeit partly
with a different set of corals, might persist.
(2) Allowing and supporting an episode of
atavistic reef growth: The future of Anthropocene
coral reefs relies on reducing emissions of CO
2
from
fossil fuels and on new forms of governance,
management and educational concepts (Leinfelder
2017) to limit the human impact on coral reefs.
However, it may also rely on protecting and
Figure 3.4.3 (cont.) BAU scenario (a):
(4a) Tipping-point phase (~2100 ~2130): episode of catastrophic tipping-point-type collapse of all coral reefs,
characterised by dying reefs.
(5a) Post-reef Anthropocene (from about mid-22nd century onwards: largely coral-free Anthropocene oceans, characterised
by biogenic and physical reef reworking.
Mitigation/management/adaptation scenario (b):
(4b) Atavisticreef phase (~20302130 CE): this is the vale of tears-adaptive episode of atavisticand other atypical
coral reefs (see text for explanation).
(5b) Full recovery phase (from ~mid-21st century onwards): renewed episode of large-scale oligotrophic Anthropocene
coral reefs, latest from ~2200 CE onwards (dotted line).
The Biostratigraphic Signature of the Anthropocene 135
Comp. by: 201508 Stage: Proof Chapter No.: 3 Title Name: Zalasiewicz
Date:18/9/18 Time:15:07:20 Page Number: 136
supporting exceptional, in part atavistic reef types
that so far thrive only in places different from the
classic modern oligotrophic reef settings and can
grow under elevated sediment and nutrient inux, as
do the Brazilian Abrolhos reefs (Leão 1982; Leinfelder
& Leão 2000; Leão & Kikuchi 2001), the Iraqi reefs off
the Shatt al Arab (Pohl et al. 2013) and the recently
discovered muddy-water reefs off the Amazon mouth
(Moura et al. 2016). In addition, many classical reefs
in the Caribbean also transform into more nutrient-
and sediment-tolerant low-diversity reef ecosystems,
which also resemble earlier reef types from Earth
history by being less rigid, more meadowlike and more
adapted to elevated coastal-runoff nutrients and heat.
This is shown by the disappearance of acroporoid, as
well as massive corals, in favor of Porites and
Siderastrea corals, soft sponges, large numbers of
brittle stars and many other changes. In the Almirante
Bay of the Caribbean of Panama, such a transition is
recorded from the 1990s (Greb et al. 1996; Berry et al.
2013; Seemann 2013; Seemann et al. 2012. Even more
unexpectedly, reef thickets and bioherms composed of
glass sponges, adapted to elevated nutrients, which
were thought to be extinct at least since the Cretaceous,
have been rediscovered alive in the PacicoffBritish
Columbia (Conway et al. 2001). Having been
threatened by sheries, they recently became fully
protected (Johnson 2017), giving such truly atavistic
reefs from the age of the dinosaurs a chance to thrive
into the future Anthropocene.
(3) Living with the vale of tears: The well-known
study on the Indonesian coral-reef triangle (Hoegh-
Guldberg et al. 2009) also concludes that reefs will
only have a chance if (a) strong management
(including new protection and social-management
schemes, as also outlined by Hughes et al. 2017b) is
implemented and (b) emissions from fossil fuels stop
around 2050. But even in this positive case, reefs
would have to cross a critical vale of tearsphase
(Figure 3.4.3b). Such specialised reefs would only
have a chance to survive if allowed to adapt to the
new conditions in a natural way or helped via human-
assisted adaptation. Such reefs would be of lower
diversity, having fewer ecological niches, and would
probably be short-lived and patchy. They would most
likely not play an important role in coastal protection
and possibly only a reduced role in supporting sh
stocks. However, after such a phase of volatility and
adaptation, reefs might hopefully recover in the late
22nd or 23rd century, possibly forming large,
structured reefs again, though different in taxon
structure relative to Holocene ones.
Combining all these aspects and fusing them into
an ecostratigraphic context, one can assume, under a
mitigation/management/assisted-adaptation
scenario, the following reef-growth-based,
correlatable ecostratigraphic episodes.
(1) The present decline phase might persist till about
20302040 CE, to be followed by a transition stage
to lower diversity, mixotrophic, managed reefs,
which frequently change their characteristics. This
means that coral reefs would continue to grow,
albeit in a completely different form, with the high-
diversity, stable communities retreating in favor of
more volatile, new, short-lived types, with some
just exchanging their key constructive elements,
while many others would show atavistic features,
such as being adapted to higher nutrient levels,
sediment runoff, deeper settings, warmer SSTs or
lower pH. All these reefs would have to be assisted
and redesigned in various ways to assist them to
go through a vale of tears possibly for the next 100,
if not 200, years (Hoegh-Guldberg et al. 2009;
Leinfelder et al. 2012; Hughes et al. 2017b).
(2) With some hope, another extended recovery phase
would set in not earlier than 2100 or 2200 CE,
with coral reefs stabilising and re-diversifying
again in clear, oligotrophic, tropical waters,
returning to robust, resilient and long-lasting
behavior, but nevertheless with a new set of coral-
reef ecologies. Such a change would provide
another easily recognisable ecostratigraphic
boundary (Figures 3.4.1 and 3.4.3b).
136 3.4 Using the State of Reefs for Anthropocene Stratigraphy: An Ecostratigraphic Approach
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19!
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Supplementary resource (1)

Chapter
Leinfelder, R. (2019): Das Anthropozän - Die Erde in unserer Hand.- In: Schwinger, E. (ed.) Das Anthropozän im Diskurs der Fachdisziplinen, . S.-23-46 Weimar bei Marburg (Metropolis-Verlag, ). (For more info see https://www.metropolis-verlag.de/Das-Anthropozaen-im-Diskurs-der-Fachdisziplinen/1394/book.do )
Preprint
Leinfelder, R. (2019, in press): Das Anthropozän - Die Erde in unserer Hand.- In: Schwinger, E. (ed.) Das Anthropozän im Diskurs der Fachdisziplinen. Weimar bei Marburg (Metropolis-Verlag, ISBN 978-3-7316-1394-7). (For more info see https://www.metropolis-verlag.de/Das-Anthropozaen-im-Diskurs-der-Fachdisziplinen/1394/book.do )
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